The use of UV transmitting acrylics has been growing each year, as a result of indoor tanning trends. The tanning operation requires a certain amount of UVB radiation (280-320 nm) to be transmitted through the acrylic sheet for effective skin tanning. The output of UV rays from a tanning lamp has been increasing to accommodate the consumer's needs for faster tanning, which adversely shortens the service life of commercial UV transparent acrylic sheet. Acrylics have been used to cover the UV lamps, since they are easy to care for, easy to fabricate, and they are one of the very few thermoplastic materials that can transmit a high percentage of the UV radiation below 300 nm region. For tanning beds, the acrylic sheet composition needs to have high UV transmission of at least 75% and preferably at least 80%, retention of UV transmission under high UV radiation, and chemical resistance. The historic problem with acrylics is that it is difficult to retain high UV transparency and clarity under strong UV radiation. The acrylic products will gradually lose their UV transmission and develop yellowness after exposing to strong UV radiation.
Several additives have been employed to stabilize acrylic compositions and retain UV transmittance.
U.S. Pat. No. 5,466,756 describes the use of an aliphatic alcohol and a high boiling hydroxyl compound in polymethylmethacrylate to improve the UV radiation resistance of the polymer.
Hindered amine light stabilizers (HALS) are described in U.S. Pat. No. 4,550,136. U.S. Pat. No. 6,716,950, and U.S. 2002/0052460 describe hindered amine light stabilizers (HALS) in combination with an additional active compound for the stabilization of acrylic resin against UV radiation. The active compounds include alcohols, water, vinyl esters, siloxanes and butyl lactate.
There is a need for acrylic composition UV stabilizers having better UV retention.
Surprisingly it has been found that the addition of a carboxylic acid compound as a UV stabilizer, and in particular an alpha-hydroxy acid compound, provides better UV stabilization to an acrylic composition than currently used stabilizers. Acrylic samples stabilized with lactic acid showed almost no loss of UV transmission and little yellowing after 3000 hours of UVB radiation, while acrylics stabilized with known stabilizers showed UV transmission losses of up to 80% of their original UV transmission.